The present invention relates generally to smart antenna technology for wireless communication systems, and more particularly to a method for calibrating smart antenna arrays, as well as to a device for calibrating smart antenna arrays.
In modern wireless communication systems, especially in CDMA wireless communication systems, smart antennas are generally used to increase system capacity, system sensitivity and communication distance with lower emission power.
The Chinese patent named xe2x80x9cTime Division Duplex Synchronous Code Division Multiple Access Wireless Communication System with Smart Antennaxe2x80x9d (CN 97 1 04039.7) discloses a base station structure for a wireless communication system with a smart antenna. The base station includes an antenna array consisting of one or more antenna units, corresponding radio frequency feeder cables and a set of coherent radio frequency transceivers. Each antenna unit receives signals from user terminals. The antenna units direct the space characteristic vectors and directions of arrival (DOA) of the signals to a baseband processor. The processor then implements receiving antenna beam forming using a corresponding algorithm. Among them, any one of the antenna unit, corresponding feeder cable and coherent radio frequency transceiver together is called a link. By using weight getting from the up link receiving beam forming of each link in the down link transmitting beam forming, the entire functionality of smart antennas can be implemented, under symmetrical radio wave propagation conditions.
In the above Chinese patent, for the smart antenna to combine receive and transmit beam accurately, the difference between each antenna unit of the smart antenna array, radio frequency feeder cable and radio frequency transceiver, should be known, i.e. the difference of amplitude and phase variation after the radio frequency signal passes each link should be known. A procedure for determining the difference among links of the smart antenna system is just one concern addressed by the smart antenna calibration of the invention.
Calibration of a smart antenna array is a kernel technology of smart antenna. A characteristic of electronic elements, which comprise radio frequency systems of smart antennas, especially active elements, is sensitivity to working frequency, environment temperature and working duration etc. Characteristics for each link, as a result of such variation, are typically never the same, thus requiring constant calibration of smart antenna systems.
At present, there are generally two kinds of calibration methods for smart antennas. One is a direct measure method. This method measures every set of radio frequency transceivers to obtain data related to its amplitude and phase. Then the measured amplitude and phase characteristics of the antenna units and feeder cables are added to form a set of calibration data. This calibration procedure is very complicated. It is difficult to obtain all measurements in the field, especially for wireless communication systems that are in operation. Another method is to calibrate the system using a pilot transceiver at an antenna far-field region. This method requires the pilot transceiver to be located at a far-field region without multipath propagation. This, however, is also difficult to implement in practice.
Therefore, an object of the invention is to provide a method and device for calibrating smart antenna arrays in real-time, so as to render the use of smart antenna systems practicable. The device of the invention allows the method of the invention to work effectively.
A further object of the invention is to provide two designs and calibration methods of couple structures for calibrating smart antenna arrays, which also allows the method of the invention to work effectively.
A method of the invention for calibrating a smart antenna array comprises:
1) providing a calibration link with a coupling structure, a feeder cable and a pilot transceiver, wherein the coupling structure is coupled with N antenna units of the smart antenna array and the pilot transceiver is connected to a baseband processor of a base station by a digital bus;
2) calibrating the coupling structure with a vector network analyzer before the smart antenna array is put into operation, and recording its receiving transmission coefficient and transmitting transmission coefficient, respectively;
3) calibrating receiving of the smart antenna array by: transmitting a defined voltage level signal at a set working carrier frequency by an analog transmitter of the pilot transceiver, and setting N receiving links, in a base station to be calibrated, in a receiving state; detecting the output of each receiving link, respectively, by the baseband processor in the base station and calculating the ratio of the transmission coefficient of each link to the transmission coefficient of a reference link during receiving, according to the output of each receiving link; controlling the output of each receiving link by controlling a variable gain amplifier in an analog receiver present in each link r, so that the amplitude ratio of the receiving transmission coefficient of each link to the transmission coefficient of the reference link equals to 1; and recording and storing the phase difference "PHgr" between each receiving link and the reference link in the baseband processor; and
4) calibrating transmitting of the smart antenna array by: setting one link in a transmitting state at one time while all other transmitting links of the N transmitting links are in a closing state, and receiving signals coming from each transmitting link, respectively, at a set working carrier frequency with an analog receiver, in the pilot transceiver; processing the signals by the baseband processor of the base station and calculating the ratio of the transmission coefficient of each link to the transmission coefficient of a reference link during transmission; controlling the output of each transmitting link by controlling a variable gain amplifier which is present in an analog transmitter in each link, so that the amplitude ratio of the transmission coefficient of each link transmission to the transmission coefficient of the reference link equals to 1, during transmission; and recording and storing the phase difference "psgr" between each transmitting link and the reference link in the baseband processor.
The method of calibrating a coupling structure with a vector network analyzer in accordance with the present invention, further comprises: setting a pilot antenna in spatial coupling mode; connecting the vector network analyzer to a feeder cable terminal of a pilot signal and antenna unit terminal of the antenna link to be calibrated, connecting an antenna unit terminal of a non-calibrated link to a matched load, measuring and recording the receiving and transmitting transmission coefficient of the link to be calibrated under each necessary working carrier frequency; and repeating the above steps until all receiving and transmitting transmission coefficients of N links have been measured and recorded.
The method of calibrating a coupling structure with a vector network analyzer of the invention further comprises: connecting a passive network coupling structure consisting of N couplers and a 1:N passive distributor/combiner, wherein the N couplers are connected with the antenna terminal of the N antenna units of the smart antenna array, respectively, and the output of the passive distributor/combiner is a feeder cable terminal of the pilot signal; connecting the vector network analyzer to a feeder cable terminal of the pilot signal and antenna unit terminal of the antenna link to be calibrated, connecting the antenna unit terminal of the non-calibrated link with matched load, measuring and recording the receiving transmission coefficient and transmitting transmission coefficient of the link to be calibrated under each necessary working carrier frequency; and repeating the steps above until all receiving transmission coefficient and transmitting transmission coefficients of N links have been measured and recorded.
The invention further includes a device for calibrating smart antenna arrays. The device comprises a calibrated coupling structure, a feeder cable and a pilot transceiver, wherein the coupling structures are coupled on N antenna units of the smart antenna array, the feeder cable is connected with the coupling structure and the pilot transceiver, and the pilot transceiver is connected to a baseband processor in the base station by a digital bus.
The coupling structure is a pilot antenna with spatial coupling mode. The pilot antenna is in the working main lobe of a radiation directivity diagram of the N antenna units, which compose the smart antenna array. The antenna terminal of the pilot antenna is a feeder line terminal of a pilot signal.
When the N antenna units, which compose the smart antenna array, are omni-directional antenna, the pilot antenna is located at any position of a near field region of each antenna unit.
The coupling structure is a passive network, wherein it includes N couplers, corresponding with the N antenna units of the smart antenna array, and a 1:N passive distributor/combiner connected with the N couplers. The N couplers are connected with antenna terminals of the N antenna units, respectively, and the output of the passive distributor/combiner is a feeder line terminal of the pilot signal.
The pilot transceiver has the same structure as the radio frequency transceiver of the base station, including a duplexer, an analog receiver connected with the duplexer, an analog transmitter connected with the duplexer, an analog-to-digital converter connected with the analog receiver and a digital-to-analog converter connected with the analog transmitter. The radio frequency interface of the duplexer is connected with the feeder cable of the coupling structure, and the analog-to-digital converter and digital-to-analog converter are connected to the digital bus.
In the analog receiver, a variable gain amplifier, controlled by software, is set for controlling gain. In the analog transmitter, a variable gain amplifier, controlled by software, is set for controlling gain.
The invention provides a method and device for calibrating smart antenna arrays using the pilot transceiver and a set of coupling structures coupled with smart antenna arrays. The coupling structure includes two technical schemes. One uses a method of calibrating a smart antenna system by a geometrical symmetric structure pilot antenna, located at near field region or far-field region, and an antenna array implementing the method, wherein the pilot antenna and related calibrating software is part of a wireless base station. The other uses a passive network consisting of couplers and distributor/combiner to implement the coupling structure feeds and calibrate the smart antenna array. Either of the two technical schemes allows easy calibration of a base station with smart antenna at all times, and allows changing radio frequency parts and elements at all times. Therefore, the invention can provide a satisfactory solution to the engineering problems associated with smart antenna systems.
The method and device of the invention for calibrating smart antenna arrays are useful in CDMA wireless communication systems. However, with simple changes the proposed method and device can also be used for calibrating smart antenna of FDMA and TDMA wireless communication systems as well.